The priority application No. 66079/1993 filed Mar. 2, 1993 is hereby incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a molded product of a polypropylene resin.
2. Description of Related Art
Polypropylene resins are thermoplastic resins which are excellent in mechanical properties such as flexural strength and modulus, and chemical resistance, as well as being inexpensive to produce. They are used to make a wide variety of molded products, such as parts of automobiles, industrial machines and electrical appliances for domestic use. However, these resins have the drawback of being very susceptible to thermal expansion and contraction, and therefore, typically result in molded products not having good dimensional stability.
A variety of improvements have been proposed in, for example Japanese Patent Applications Laid-Open Nos. 21145/1986, 34048/1986, 153334/1987, 186754/1988, 271450/1989 and 311533/1990, and Japanese Patent Publication No. 2620/1992 all of which are incorporated herein by reference. The polypropylene resins disclosed therein have, however, a coefficient of linear expansion in the range of 5.0.times.10.sup.-5 to 9.0.times.10.sup.-5 cm/cm/.degree.C. Therefore, they are still very liable to thermal expansion and contraction, and difficult to employ for producing molded products having good dimensional stability, particularly with respect to producing molded slender parts, such as side moldings, locker moldings, side garnishes, lower door moldings and bumpers for automobiles.
We, the inventors of this invention, have, therefore, engaged in a wide range of research and experimental work to obtain a polypropylene resin having a lower coefficient of linear expansion than the prior art. Consequently, the inventors have earlier developed a molded product having a coefficient of linear expansion in the range of 3.0.times.10.sup.-5 to 3.9.times.10.sup.-5 cm/cm/.degree.C., and having a high level of dimensional stability, as disclosed in Japanese Patent Application Laid-Open No. 8744/1992 incorporated herein by reference. This product is formed from poly-propylene, an amorphous ethylene-.alpha.-olefin copolymer and a fibrous filler. Its polymer component comprises 30 to 70 parts by weight of a polypropylene resin (PP) and 30 to 70 parts by weight of an amorphous ethylene-.alpha.-olefin copolymer (such as EPR) having a Mooney viscosity of less than 70 and occupies 60 to 85% by weight of the product, while EPR has an aspect ratio of 5 or more and is distributed in a flat plane to enable a lower coefficient of linear expansion. The product has, however, surface which is more sticky than that of the typical product containing PP, EPR and a fibrous filler. If it is used to make a product of injection molding not having an embossed surface, but having a mirror surface, the product tends to stick to the mold and is difficult to release, and consequently is likely to be deformed or damaged.
The inventors have therefore, considered a number of modifications to obtain a molded product of a polypropylene resin which is easy to release from a mold. They include:
(1) The use of a longer cooling time; PA1 (2) The application of a mold release agent to the mold; PA1 (3) The use of EPR in a lower proportion; PA1 (4) The use of EPR having a higher molecular weight; PA1 (5) The use of the fibrous filler in a higher proportion; and PA1 (6) The use of two or more kinds of EPR having different compositions and physical properties. PA1 (1) The use of a longer cooling time results in a longer molding cycle; PA1 (2) The application of a mold release agent disables an automatic molding operation; PA1 (3) and (4) The use of EPR in a lower proportion, or EPR having a higher molecular weight results in a resin having a high coefficient of linear expansion and consequently a molded product of low dimensional stability; PA1 (5) The use of the fibrous filler in a higher proportion brings about a reduction in impact strength and moldability; and PA1 (6) Although he use of two or more kinds of EPR is disclosed in the prior publications as mentioned hereinabove, none of them contains any suggestion that enables the improvement of both the dimensional stability of a molded product and its releasibility form the mold.
These modifications have, however, been found undesirable for the reasons stated below.